Extraction to ocaml is now working, with a couple of bugs left.
One limitation is that it is not possible to have two files with the
same name in different directories. Therefore this commit renames files
to avoid this situation.

The extracted directory contains:

a snapshot of the .ml(i) files extracted from CerCo? by running
ocamlc.opt -extract_ocaml compiler.ma
The files have been patched by hand to implement all strings and
fix the bugs.

a file PROBLEMS that describes the remaining problems, i.e. bugs
and axioms to be implemented

To obtain the compiler, run ocamlbuild compiler.byte.
At the moment it fails because of the three remaining axioms.